Ultrasonics is basically the science of the effects of sound vibrations beyond the limit of audible frequencies. The object of high power ultrasonic applications is to bring about some permanent physical change in the material treated. This process requires the flow of vibratory energy per unit of area or volume. Depending on the application, the resulting power density may range from less than a watt to thousands of watts per square centimeter. Although the original ultrasonic power devices operated at radio frequencies, today most operate at 20-69 kHz.
Ultrasonics is used in a wide variety of applications. For example, ultrasonics is used for (1) dust, smoke and mist precipitation; (2) preparation of colloidal dispersions; (3) cleaning of metal parts and fabrics; (4) friction welding; (5) the formation of catalysts; (6) the degassing and solidification of molten metals; (7) the extraction of flavor oils in brewing; (8) electroplating; (9) drilling hard materials; (10) fluxless soldering and (11) nondestructive testing such as in diagnostic medicine.
Ultrasonic vibratory forces have also been used in the areas of welding textile sheet materials. U.S. Pat. No. 3,697,357 to Obeda discloses welding sheets made entirely or partially of thermoplastic material or fiber. Obeda discloses sealing an area of material by placing it between an anvil and an ultrasonic horn. U.S. Pat. No. 3,939,033 to Grgach discloses an ultrasonic apparatus for the simultaneous sealing and cutting of thermoplastic textile material. U.S. Pat. No. 5,061,331 to Gute discloses another ultrasonic cutting and edge sealing apparatus suitable for cutting and sealing semipermeable and at least partially thermoplastic fabric.
Ultrasonic vibratory force has also been used to perforate or aperture sheet materials. U.S. Pat. No. 3,966,519 to Mitchell et al. discloses a method of aperturing nonwoven webs. Mitchell teaches that the amount of ultrasonic energy to which a nonwoven web is subjected can be controlled by applying enough of a fluid to the area at which the ultrasonic energy is applied to the nonwoven web so that the fluid is present in uncombined form. Mitchell teaches that the fibers of the nonwoven web rearrange to form apertures in the web.
U.S. Pat. No. 3,949,127 to Ostermeier discloses a method of aperturing nonwoven webs by submitting the web to intermittent ultrasonic fusion and then stretching the web to break the most intensely fused regions causing perforations to form in the web.
Ultrasonic force has also been used to aperture nonporous film material. U.S. Pat. No. 5,269,981 to Jameson, et al. discloses a method of microaperturing thin sheet material which requires applying a liquid to the thin film before subjecting it to ultrasonic vibrations. Jameson further discloses that inclusion of the liquid in the process is essential. Without the liquid, Jameson teaches, that the process is not successful because the thin film is melted without aperturing the film. Jameson teaches that it is believed that the presence of the fluid during operation of the ultrasonic horn accomplishes two separate and distinct functions. First, the presence of the fluid allows the fluid to act as a heat sink which allows the ultrasonic vibrations to be applied to the thin sheet material without the thin sheet material being altered or destroyed by melting. Second, the presence of the fluid allows the fluid to act as a coupling agent in applying the vibrations from the ultrasonic horn to the thin sheet material.
Aperturing thin sheets using ultrasonics is desirable because it allows rapid movement of the thin sheet through the process without creating waste. In the past, methods used to aperture thin sheets have included punching the sheet material. Although punching the aperture out of the sheet material created the desired perforated effect, punching left behind residual waste in the form of cores. The cores often adhered to the thin sheet material and produced an undesirable effect because they interfered with manufacturing of the final product.
Another method of aperturing thin sheets included passing the thin sheet material through a patterned heated roller. This method ultimately melted the apertures into the thin sheet material resulting in an adequately apertured material without additional waste adhering to the thin sheet. Even though it overcame the problems caused by punching the thin sheet, the heated roller method was a slow process. In the nip created between the heated roller and the patterned anvil, the thin sheet material was heated and the apertures were formed in the thin sheet material. The thin sheet material was then cooled to prevent the apertures from melting back together. All of these heating, forming and cooling processes took time and it was required that these processes take place in the narrow area of the nip. Thus, the process proceeded relatively slowly and this method was not a viable method of aperturing thin sheets.
A method of aperturing thin sheet materials is needed which is relatively rapid compared to the heated roller method and is relatively waste-free as compared to the punching method. A method of perforating webs which utilizes ultrasonics overcomes the shortcomings of the previous perforating methods. The present invention uses ultrasonics to perforate webs and obviates the need to stretch the web and also obviates the need to use liquid in combination with the ultrasonic vibrations.